playcanvas/build/playcanvas.dbg/src/scene/shader-lib/wgsl/chunks/internal/frag/packHalf.js

PlayCanvas WebGL game engine

// Generic half-float packing with software fallback for subnormals
// Addresses vendor differences in pack2x16float subnormal handling (e.g., Adreno (TM) 750 on Samsung Galaxy S24)
var packHalfPS = /* wgsl */ `

#if defined(PLATFORM_ANDROID)

    // Software pack of one f32 -> f16 (low 16 bits). Ties-to-even, full subnormals.
    fn floatToHalf(a: f32) -> u32 {
        let u: u32    = bitcast<u32>(a);
        let sign: u32 = (u >> 16u) & 0x8000u;
        let absu: u32 = u & 0x7FFFFFFFu;
        let man: u32  = u & 0x007FFFFFu;
        let e32: i32  = i32((u >> 23u) & 0xFFu) - 127;
        
        // NaN / Inf
        if ((absu & 0x7F800000u) == 0x7F800000u) {
            let isnan = (man != 0u);
            return sign | select(0x7C00u, 0x7E00u, isnan);
        }
        
        // Overflow to Inf
        if (e32 > 15) { return sign | 0x7C00u; }
        
        // Normal half
        if (e32 >= -14) {
            var he: u32 = u32(e32 + 15);
            var hm: u32 = man >> 13u;
            let rem: u32 = man & 0x1FFFu;
            let add: u32 = select(0u, 1u, (rem > 0x1000u) || (rem == 0x1000u && (hm & 1u) == 1u));
            hm = (hm + add) & 0x3FFu;
            if ((hm & 0x400u) != 0u) {
                hm = 0u; he = he + 1u;
                if (he >= 31u) { return sign | 0x7C00u; }
            }
            return sign | (he << 10u) | hm;
        }
        
        // Subnormals
        if (e32 >= -24) {
            let s: u32      = u32(-(e32 + 1));
            let mnorm: u32  = 0x00800000u | man;
            var hm: u32     = mnorm >> s;
            let mask: u32   = (1u << s) - 1u;
            let rem: u32    = mnorm & mask;
            let halfBt: u32 = 1u << (s - 1u);
            let add: u32    = select(0u, 1u, (rem > halfBt) || (rem == halfBt && (hm & 1u) == 1u));
            hm = hm + add;
            if (hm >= 0x400u) { return sign | (1u << 10u); }
            return sign | hm;
        }
        
        return sign; // signed zero
    }

    // Hybrid pack: software for subnormals, builtin for normal range
    fn pack2x16floatSafe(v: vec2f) -> u32 {
        // Convert the input floats to their 32-bit IEEE-754 bit patterns.
        // We'll inspect the exponent bits directly to determine their numeric range.
        let u_x: u32  = bitcast<u32>(v.x);
        let u_y: u32  = bitcast<u32>(v.y);
        
        // Extract the unbiased exponent for each component (float32 uses bias = 127).
        // e32 = exponent - 127  ⇒  actual power of two for each value.
        let e32_x: i32 = i32((u_x >> 23u) & 0xFFu) - 127;
        let e32_y: i32 = i32((u_y >> 23u) & 0xFFu) - 127;
        
        // -------------------------------------------------------------------------
        // Detect values that would become *subnormal* (or zero) in float16.
        //
        //   e32 < -14  ⇔  |value| < 2^-14 ≈ 6.1035e-5
        //
        // Many mobile GPUs (including Adreno and Mali) mishandle half-precision
        // subnormals—typically flushing them to zero or rounding incorrectly.
        // To preserve correct rounding and sign, we use the software conversion
        // path (floatToHalf) for these small magnitudes.
        //
        // The software branch runs very rarely (<0.1% of typical values for
        // normalized scene data) and costs only a few ALU instructions, so the
        // performance impact is negligible while avoiding visible precision loss.
        // -------------------------------------------------------------------------
        if (e32_x < -14 || e32_y < -14) {
            // Convert both components with the reference software routine
            // and pack into a 32-bit uint: low 16 bits = x, high 16 bits = y.
            return (floatToHalf(v.y) << 16u) | floatToHalf(v.x);
        }
        
        // Normal range: use the fast hardware builtin
        return pack2x16float(v);
    }

#else

    // On non-Android platforms, use builtin directly (no subnormal workaround needed)
    fn pack2x16floatSafe(v: vec2f) -> u32 {
        return pack2x16float(v);
    }

#endif
`;

export { packHalfPS as default };